Page last updated at 09:53 GMT, Wednesday, 29 April 2009 10:53 UK

Sun-like star's 'oddball' planet

Simulation of atmosphere of HD80606b (
The Jupiter-sized planet is in an eccentric orbit

Astronomers have discovered a strange Jupiter-sized world circling a star similar to our own Sun.

The planet has a highly unusual, elliptical orbit around its parent star.

At its furthest point, the planet is about as far from its star as the Earth is from the Sun.

But at its nearest, it is about 10 times closer to its star than Mercury is to the Sun, the team told the JENAM 2009 conference in Hertfordshire.

The planet, HD80606b, makes its close approach every 111 days, according to the study.

If an observer were to hover above the cloud tops of this world, they would see their parent star grow to 30 times the size that the Sun appears in our own sky.

The team from University College London observed a transit, in which the strange world crossed in front of its parent star.

The data gathered from this event provided the astronomers with the most precise data yet on the planet's size and density, its tilt and the eccentricity of its orbit.

Record breaker

For example, the transit shows that the planet has a radius about the same as Jupiter - despite being about four times more massive.

HD80606b would now appear to hold the record for both the longest orbital period and most eccentric orbit of all observed transiting planets.

At its closest approach, the planet comes within five million kilometres (three million miles) of its star. At its furthest point, the planet is about 132 million kilometres (82 million miles) away.

"The temperature on the planet is changing from about 3C - which is what you might have on Earth - to about 1,200C. So it is going through a huge change in the amount of heating," co-author David Kipping, from UCL, told BBC News.

Team leader Dr Steve Fossey commented: "Spectroscopic observations reported by a French-Swiss team, when combined with our precise measurement of the orbital tilt, indicate that the planet's unusual orbit might be explained by the parent star being a member of a binary system."

This, he said, was where "the companion star tugs on the planet's orbit over millions of years to leave it in the strange configuration we see today".

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